Method of forming magnetic cores



Nov. 28, 1961 Filed Oct. 21, 1958 R. E. HUME METHOD OF FORMING MAGNETIC CORES 2 Sheets-Sheet 1 z/ /7 i 7" 9 5 -J/ 1 \fi 3 /;A JPF LJ, 13- 4 [Z 25 IL I 1 r4 L 2a A r L 2. A /,a l2

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fitter-my 3,010,185 Patented Nov. 28, 1961 United States Patent Ofiice 3,010,185 METHOD OF FORMING MAGNETIC CORES Roger E. Hume, Danville, 111., assignor to General Electric Company, a corporation of New York Filed Oct. 21, 1958, Ser. No. 768,696 5 Claims. (Cl. 29-155.58)

This invention relates to magnetic cores for inductive devices and to a method of forming the same, and more particularly to laminated magnetic cores for high reactance type transformers such as are used for ballasts for electric discharge devices, and to improvements there in, and to a method of forming the laminations therefor.

Core constructions of the multiple coil type transformers, having a number of separate coils, frequently need to provide different values of reactance for the coils, and consequently different reluctance paths for the flux of the different coils. Such different reluctance paths may be introduced into the core, for example, by variationsfin the spacings of the windings relative to each other, by the introduction of magnetic shunts between the windings, or by restricted cross sectional areas of the core. Such restricted cross sectional areas may be formed, for example, by a slot in the core and are commonly termed bridged air gaps.

Such cores are generally constructed in laminated form, with each lamination layer being formed of two or more pieces to constitute winding and yoke members, with the above mentioned variations in the magnetic circuit to provide the necessary variations in the reluctance paths. Such laminations are generally punched from metal strip or sheet. It is desirable that for a given electrical size or rating of the transformer, the gross area of the strip of material from which the core lamination punchings are made be a minimum, and that the simpler yoke section.

percentage of scrap or loss of strip material from which 7 the core laminations are punched is also at a minimum. When it is realized that cores of this type are made by the hundreds of thousands, it will be appreciated that such savings are highly important.

One present form of ballast used for starting and operating two series connected fluorescent lamps comprises at least two windings, for example, a primary winding, a first or ballasting secondary winding on one side of the primary winding, and a second or starting secondary winding on the other side of the primary winding, all wound on a center winding carrying leg member of the laminated core; the core further comprises two oppositely located yoke members which complete the inductive path. A' magnetic shunt path is located between the primary winding and the starting winding to provide loose inductive coupling of the primary winding and the starting winding. This type of ballast is, for example, shown in U.S. Patent No. 2,558,293 issued June 26, 1951 to A. E. Feinberg and is particularly useful in the circuits disclosed therein.

The need for the aforesaid shunt path presents particular difficulty in arranging the punching dies which form the core laminations so that there results a minimum amount of waste of the sheet material from which the punchings are made and which further results in the maximum utilization of the core material in the magentic path.

Normally the center leg of the core also contains a slot forming a bridged air gap under the ballasting wind ings. Such a bridged gap provides improved lamp current wave form as compared to the wave form provided by ballasts having a full air gap or no gap at all. Also as compared to a full air gap, it produces an increase in the R.M.S. voltagefor a given number of turns in the coil, as well as providing a one piece center leg and a The provision of the slot for the bridged air gap, however, requires a specific section in the punching die for that purpose, and the elimination of this slot would'result in simpler punching dies having reduced maintenance and longer life.

In order to obtain a reduction in the scrap and the maximum utilization of the core material, the yoke and leg laminations may be punched together from a single strip of material; however, the formation of the magnetic shunt in the yoke of the core then necessitates the punching of a notch in the center leg member of the core. Certain d-itficulties are experienced when the notch in the center leg is in the magnetic circuit of, or under, the starting winding in that the introduction of the notch decreases the inductance of the starting coil; the effect of the notch may be particularly adverse when one of two ballasted lamps operated by the transformer is out. It is therefore desirable that the notches in the center leg be so arranged elsewhere so as to be more beneficial to the operation of the circuit. 7

An object of the invention is to provide an improved magnetic core for use in a ballasting transformer.

A further objectis to utilize advantageously the notch formed in the center leg of the core by the magnetic shunt in the yoke section when the center leg and the yoke are formed together from the same strip of material.

Another object is to provide an improved method of forming a transformer of the type above described.

Another object is to provide an improved method of forming a magnetic core for an inductive device.

Another object of this invention is to provide a core of the type described wherein the laminations are so formed that they may be stamped out of strip material with a minimum waste and a maximum utilization of strip material in the magnetic circuit.

Briefly stated, by my invention there is provided a laminated core for a transformer of the type above described. The laminations of the core are formed by punching the yoke laminations from the edges of a strip of material simultaneously with the punching or forming of the leg laminations from the center of the strip. The formation of the magnetic shunts of the yoke laminations during punching causes the formation of notches in the center legs. The core is formed by stacking a plurality of yoke laminations and a pluraiity of leg laminations, the leg laminations being turnedend-for-end with respect to the yoke laminations, and bringing the yoke and leg members into assembled relation. As a result of the reversal of the center leg during stacking, the notches in the finished core are located at least partially under the ballasting winding so as to be in the magnetic circuit thereof. Thus, there is provided a transformer having a very advantageous magnetic circuit, with the laminations being formed by relatively simple dies and with there being a minimum of scrap material.

The invention together with additional objects and advantages thereof Will be best understood from the following description of specific embodiments, when read in connection with the accompanying drawings, in which:

FIG. 1 is a pictorial view of an assembled laminated core according to one embodiment of the invention;

FIG. 2 is a cross-sectional view of a core with windings thereon according'to the embodiment of FIG. 1;

FIG. 3 is a plan view of the layout of the laminations for the embodiment of FIG. 1 as they are punched from the strip of material;

FIG. 4 is a pictorial View of an assembled laminated core according to another embodiment of the invention;

FIG. 5 is a cross-sectional view of a core with windings thereon according to the embodiment of FIG. 4; and

FIG. 6 is a plan view of the layout of the laminations for the embodiment of FIG. 4 as they are punched from the strip of material.

Referring to the embodiment of FIGS. 1, 2 and 3, there is illustrated a ballasting transformer core comprising a generally T-shaped center winding leg member 1 and generally L-shaped yoke members 2, 3 cooperating with the leg 1. The center leg and yokes are each formed of a plurality of laminations stacked to form the respective members and comprising laminations 1 1 1 etc.; 2,, 2 2 etc.; 3 3 3 etc.

The ballasting transformer has wound on the center winding leg a primary winding 4; a first or ballasting secondary winding 5; and a second or starting secondary winding 6. The windings are generally formed by winding the necessary turns of wire 7 on an insulating sleeve 8. Such preformed windings may then be slipped over the center leg 1. Magnetic shunts 9, 10 extend integrally from the L-shaped yoke members between the primary winding 4 and the starting winding 6 so as to result in loose inductive coupling of these windings. The final assembled core is clamped or otherwise securely held together.

As previously explained, it is desirable to have a difference in the reluctance of the magnetic paths of the primary winding 4 and ballasting secondary winding 5; according to the invention notches 11, 12 are placed in the center leg at least partially under the ballasting winding 5; as appears in the embodiment of FIGS. 1 to 3, the notches appear wholly under the ballasting winding 5. As will be readily appreciated, the notches in the center leg replace a slotted air gap in the magnetic circuit of the ballasting winding, the notches forming, in effect, a bridged air gap. This ballasting transformer with its primary and two secondaries, one more loosely coupled than the other, is particularly useful in the circuitry of the above mentioned Feinberg patent.

FIG. 3 illustrates the punching of the laminations from a strip of sheet material having the laminations arranged so as to provide maximum utilization of the sheet material. For the formation of the laminations, the center leg lamination is formed end-for-end with respect to the final assembled position of the leg member with the yoke member, and positioned substantially within the yoke;

the yoke laminations cooperating substantially with the leg laminations so that the leg laminations substantially engages the adjacent sides of the yoke laminations and the projecting shunts substantially engage the notches. It will be noted that as the yoke laminations 2 3 are formed from the strip with their integral shunts 9, 10, the notches 11, 12 are formed in the center leg laminations by the removal of the shunts. The center leg punching is not, however, struck out entirely from within the yoke laminations. Rather, at one end 17 the center leg punching extends beyond the yoke punchings, and at the other end 18 the yoke punchings extend beyond the center leg and inwardly toward each other as indicated at 19 and 20. The cross-head of the T-shaped center leg punching is cut out from the notches 15, 16 of the yoke punchings. Very small areas of scrap are created at 21, 22 and 23 but the areas are of minimum size.

To form the core, a plurality of the yoke alminations are stacked to form the yoke members, a plurality of leg laminations are stacked to form the leg member, the leg and yoke members are turned end-for-end with respect to each other either before or after stacking, and the stacks of yoke laminations and leg laminations are brought into assembled relation with each other.

It will be understood that prior to the assembly of the yoke members with the center leg, the primary winding the ballasting secondary winding and the starting secondary winding are placed on the center leg member, with a secondary winding on each side of the primary winding and spaced apart therefrom; specifically, a space is formed between the starting secondary winding and the primary winding. When the yoke members are then assembled with the center leg, the magnetic shunts 9, 10 formed on the yoke members extend into the space between the primary winding 4 and the starting secondary winding 6.

Referring now to the modification of FIGS. 4, 5 and 6, there is illustrated a ballasting transformer core hav ing a T-shaped center leg 24 and L-shaped yokes 25, 26 on each side of the center leg 24. The core further con-' tains integral magnetic shunts 27, 28 and the notches 29, 3G in the center leg. As previously stated in discussing the embodiment of FIGS. 1, 2 and 3, the core is built up of a plurality of laminations stacked together.

The center leg member in the final assembled transformer contains a primary winding 31, a ballasting secondary winding 32, and a starting secondary winding 33. The magnetic shunts 27, 28 formed on the yoke members 25, 26 are located between the primary winding 31 and the starting secondary winding 33 so as to loosely couple inductively the respective windings. The notches 29, 30 in the center leg 24 appear partly under the primary winding 31 and partly under the ballasting secondary winding 32; the notches 29, 30 function to restrict the magnetic circuit in this path thereby varying the reluctance of the magnetic circuit of the primary winding 31 and the ballasting secondary 32, as previously explained.

The laminations are arranged for punching to result in a minimum scrap and maximum utilization of the sheet material from which the laminat'ons are punched, as illustrated in FIG. 6. The punchings consist of a center leg lamination 24,, formed endto-end with respect to its cooperating yoke laminations 25 26 as compared to its assembled position in the transformer, and positioned generally within the yoke laminations.

It will be noted that as the yoke laminations 25,,, 26 are formed from the strip with the integral shunts 27, 23, the notches 2?, 30 are formed in the center leg lamination by the removal of the shunts. The center leg punching is not, however, struck out entirely from within the yoke laminations. Rather, at one end 35 the center leg punching extends beyond the yoke punchings, and at the other end 36 the yoke punchings extend beyond the center leg and inwardly toward each other as indicated at 37 and 38. Very small areas of scrap are created at 39, 40 and 41 but the areas are of minimum size. In this position, the yoke laminations 25 26 cooperate substantially with the leg lamination 24 so as to substantially engage the adjacent side of the center leg; and the magnetic shunts 27, 28 substantially engage the notches 29, 30.

It will be noted that in the instant embodiment, the core contains cult-out portions 42, 43 in the yoke members formed by cutting out the crosshead of the center leg 24 and also includes cut-out portions 44, 45 in the center leg; however, these restricted cut-out portions are at the end of the ballasting secondary 32 most remote from the primary winding 31 and their efiect upon the operation of the ballast is not material. At the same time, a ballasting transformer containing a core according to this embodiment has the further advantages heretofore mentioned and overcomes the ditficulties heretofore discussed.

In accordance with the patent statutes, I have described what at present are considered to be the preferred embodiments of my invention. However, it will be obvious to those skilled in the art that various changes and modifications may be made in the disclosed structure without departing from my invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of forming a magnetic core for an inductive device of the type comprising a center Winding leg member composed of a plurality of leg laminations defining a crosshead at one end thereof and a yoke member at each side of said leg, each of said yoke members being composed of a plurality of yoke laminations, comprising'thesteps of forming from a strip of material yoke laminations containing integral magnetic shunts projecting therefrom and simultaneously forming said center leg laminations having notches therein and said crosshead being positioned on said strip substantially within said yoke laminations and cooperating with said yoke laminations whereby said center leg laminations substantially engage the adjacent sides of said yoke laminations and whereby the formation of said projecting shunts forms said notches, stacking a plurality of said yoke laminations, stacking a plurality of said leg laminations, turning said leg laminations and yoke laminations end-forend with respect to each other, and bringing said plurality of yoke laminations into assembled relation with said plurality of leg laminations whereby said core member is formed.

2. The method of forming a ballasting transformer of the type having a center leg defining a crosshead at one end thereof and containing at least two windings, and a yoke section on each side of said center leg comprising the steps of forming from a strip of material yoke laminations containing integral magnetic shunts projecting therefrom and simultaneously forming said center leg having notches therein and said crosshead being positioned on said strip substantially within said yokes and cooperating withsaid yokes whereby said center leg substantially engages the adjacent sides of said yokes and whereby the formation of said projecting shunts forms said notches, the end opposite said crosshead extending outwardly of said yoke sections, stacking a plurality of said yoke laminations, stacking a plurality of said leg laminations, placing the said windings on said leg spaced apart from each other so as to form a space between said windings, turning said leg laminations and yoke laminations end-forend with respect to each other, and bringing said plurality of yoke laminations into assembled relation with said plurality of leg laminations with the shunts of said laminations extending into said space between said windings.

3. The method of forming a ballasting transformer of the type having a center leg member having a crosshead at one end thereof and containing a primary winding, a ballasting secondary winding, and a starting secondary winding, and a yoke member on each side of said center leg member comprising the steps of forming from a strip of material yoke laminations containing integral magnetic shunts projecting therefrom and simultaneously forming center leg laminations having notches therein and said crosshead being positioned on said strip substantially within said yoke laminations and cooperating with said yoke laminations whereby said center leg laminations substantially engages the adjacent sides of said yoke laminations and whereby the formation of said projecting shunts forms said notches, the end of said center leg laminations opposite said crosshead extending outwardly of said yoke laminations, stacking a plurality of said yoke laminations to form said yoke members, stacking a plurality of said leg laminations to form said leg member, placing the ballasting winding on said center leg member at least partially over said notches, placing the primary winding on said leg on one side of said ballasting winding, and placing the starting winding on said leg member on the other side of said primary winding from said ballasting winding and spaced apart from the primary winding to form a space between said primary winding and said starting winding, turning said leg laminations and yoke laminations end-for-end with respect to each other so that said outwardly extending end of said center leg laminations define a reduced section under one of said secondary windings, and bringing said plurality of yoke laminations into assembled relation with said plurality of leg laminations with the shunts of said laminations extending into said space between the primary winding and the starting winding.

4. A method of producing a magnetic core assembly having a center core section formed of a stack of T-shaped laminations and an outer core section formed of a stack of L-shaped laminations, said T-shapecl laminations having a crosshead formed of two outwardly extending projections, a pair of oppositely disposed notches and a reduced end portion; said L-shaped laminations having a first leg of a predetermined Width extending outwardly at one end, a second leg extending outwardly at the other end, a notch adjacent said first leg and an outwardly extending projection located between said notch and said second leg; and said T-shaped and L-shaped lamina tions having side parts of complementary configuration Where said side parts are contiguous when said pair of L-shaped laminations are oppositely disposed along said side parts of said T-shaped lamination and arranged so that the crosshead of said T-shaped lamination engages said notches formed in said L-shaped lamination and said reduced portion of said T-shaped lamination protrudes for a length equal to the predetermined width of said first leg of the L-shaped lamination, said method comprising the steps of: stamping said T-shaped and L-shaped laminations as a group of two L-shaped and one T-shaped laminations in a continuous process from a sheet of magnetic material, said T-shaped and L-shaped laminations being disposed so that their side parts of complementary configurations are contiguous; and assembling said L-shaped laminations and T-shaped laminations so that said first legs of the L-shaped laminations abut said reduced end portions of the T-shaped laminations and said second legs abut against the ends of the crossheads of said T-shaped laminations whereby a core assembly is formed having two restricted sections and a magnetic shunt.

5. A method of producing a magnetic core assembly having a center core section formed of a stack of T-shaped laminations and an outer core section formed of a stack of L-shaped laminations, said T-shaped laminations having a crosshead formed of two outwardly ex tending projections, a pair of oppositely disposed notches and a reduced end portion; said L-shaped laminations having a first leg of a predetermined width extending outwardly at one end, a second leg extending outwardly at the other end, a notch adjacent said first leg and an outwardly extending projection located between said notch and said second leg; and said T-shaped and L-shaped laminations having side parts of complementary configuration where said side parts are contiguous when said pair of L-shaped laminations are oppositely disposed along said side parts of said T-shaped lamination and arranged so that the crosshead of said T-shaped lamination engages said notches formed in said L-shaped lamination and said reduced portion of said T-shaped lamination protrudes for a length equal to the predetermined width of said first leg of the L-shaped lamination, said method comprising the steps of: stamping said T-shaped and L-shaped laminations in a continuous process from a sheet of magnetic strip material, said laminations being stamped as a group of a pair of L-shaped laminations and a T-sha-ped lamination, said T-shaped lamination having an L-shaped lamination disposed on the side parts of the T-shaped lamination and having said crosshead cut out from said notches of the pair of L-shaped laminations and said outwardly extending projections of the L-shaped laminations are cutout from said oppositely disposed notches of the T-shaped lamination and thereafter assembling said L-shaped and T-shaped laminations by arranging the T-shaped laminations so that they are turned end-forend with respect to the L-shaped laminations, the ends of said crossheads of the T-shaped laminations abutting against the ends of said legs of L-shaped laminations and said first legs of the L-shaped' laminations abutting against the reduced portion of the T-shaped lamination whereby a magnetic core is formed having two restricted cross sections and a magnetic shunt.

References Cited in the file of this patent UNITED STATES PATENTS Somerville Dec. 14, Mitterrnaier Jan. 10, Clark Oct. 15, Henderson Mar. 18, Feinberg et a1 June 30, Dierstein Oct. 6, 

